Selection using Switch Statements

In previous lesson we saw how we can use "if" statement  in programs when they need to choose an option from among several alternatives.  There is an alternative C programming command which in some cases can be used as an alternate way to do this. “Switch…case” command is more readable and easier language structure.

"Switch ... case" structure

We can use "if" statement yet but it is better to use "switch" statement which is created for situations that there are several choices.

switch(...)
 {
  case ... : command;
             command;
             break;

  case ... : command;
             break;

  default:
             command;
 }

In the above switch command we will be able to run different series of commands with each different case.

Example 5-1: example5-1.c

Rewrite example 4-5 of previous lesson and use switch command instead of "if" statement.

#include<stdio.h>
#include<stdlib.h>
main()
{
int choice;

while(1)
  {
  printf("\n\nMenu:\n");
  printf("1- Math Program\n2- Accounting Program\n");
  printf("3- Entertainment Program\n4- Exit");
  printf("\n\nYour choice -> ");
  scanf("%d",&choice);
  switch(choice)
    {
     case 1 : printf("\nMath Program Runs. !");
              break;
     case 2 : printf("\nAccounting Program Runs. !");
              break;
     case 3 : printf("\nEntertainment Program Runs. !");
              break;
     case 4 : printf("\nProgram Ends. !");
              exit(0);
     default:
              printf("\nInvalid choice");
    }
  }
}

In "switch…case" command, each “case” acts like a simple label. A label determines a point in program which execution must continue from there. Switch statement will choose one of “case” sections or labels from where the execution of the program will continue. The program will continue execution until it reaches “break” command.

"break" statements have vital rule in switch structure. If you remove these statements, program execution will continue to next case sections and all remaining case sections until the end of "switch" block will be executed (while most of the time we just want one “case” section to be run).

As we told, this is because each “case” acts just as a label. The only way to end execution in switch block is using break statements at the end of each “case” section.

In one of case sections we have not used "break".  This is because we have used a termination command "exit(0)" (which terminates program execution) and a break statement will not make any difference. 

"default" section will be executed if none of the case sections match switch comparison.

Parameter inside “switch” statement must be of type “int or char” while using a variable in “case sections” is not allowed at all. This means that you are not allowed to use a statement like below one in your switch block.

  case i:   something; 
            break;

Break statement

We used "break" statement in “switch...case” structures in previous part of this lesson. We can also use "break" statement inside loops to terminate a loop and exit it (with a specific condition).

Example 5-2: example5-2.c

while (num<20)
 {
  printf("Enter score : ");
  scanf("%d",&scores[num]);

  if(scores[num]<0)
     break;
  }

In above example loop execution continues until either num>=20 or entered score is negative. Now see another example.

Example 5-3: example5-3.c

#include<stdio.h>
#include<stdlib.h>

main()
{
int choice;
while(1)
  {
  printf("\n\nMenu:\n");
  printf("1- Math Program\n2- Accounting Program\n");
  printf("3- Entertainment Program\n4- Exit");
  printf("\n\nYour choice -> ");
  scanf("%d",&choice);

  switch(choice)
    {
     case 1 : printf("\nMath Program Runs. !");
              break;
     case 2 : printf("\nAccounting Program Runs. !");
              break;
     case 3 : printf("\nEntertainment Program Runs. !");
              break;
     case 4 : printf("\nProgram Ends. !");
              break;
     default:
              printf("\nInvalid choice");
    }

  if(choice==4) break;
  }
}

In above example we have used a break statement instead of exit command used in previous example. Because of this change, we needed a second break statement inside while loop and outside switch block.

If the choice is 4 then this second “break command” will break while loop and we reach the end of main function and when there is no more statements left in main function program terminates automatically.

getchar()  and getch()

getchar() function is an alternative choice when you want to read characters from input. This function will get single characters from input and return it back to a variable or expression in our program.

  ch=getchar();

There is a function for sending characters to output too.

  putchar(ch);

Example 5-4: example5-4.c

#include<stdio.h>
#include<conio.h>
main()
{
char ch;

while(ch!='.')
 {
  ch=getchar();
  putchar(ch);
 }

system("pause");
}

First look at output results and try to guess the reason for results. Console Screen:

test            <--This is typed by us
test            <--output of putchar after we pressed enter
again.testing   <--Typed string includes a '.' character
again.          <--Loop terminates when it reaches '.' char

Above program reads any character that you have typed on keyboard until it finds a '.' character in input characters. Each key press will be both shown on console and added to a buffer. This buffer will be delivered to 'ch' variable after pressing enter key (not before this). So input characters are buffered until we press enter key and at that moment program execution continues running statements that follow getchar() function (These are loop statements).

If there is a '.' character in buffered characters, loop execution continues sending characters to console with putchar() function until it reaches '.' and after that  stops the loop and comes out of while loop.

If it does not encounter '.' in characters it returns to the start of loop, where it starts getchar() function again and waits for user input.

First 'test' string appeared in output is the result of our key presses and second 'test' is printed by putchar statement after pressing enter key.

In some operating systems and some C Programming language compilers, there is another character input function "getch". This one does not buffer input characters and delivers them as soon as it receives them. (in Borland C you need to include another header file <conio.h> to make this new function work).

Example 5-5: example5-5.c

#include<stdio.h>
main()
{
char ch;

while(ch!='.')
 {
  ch=getch();
  putch(ch);
 }

system("pause");
}

Testing.        <--program terminates immediately after we use the
                character ‘.', also characters are sent to output once

With this function we can also check validity of each key press before accepting and using it. If it is not valid we can omit it. Just pay attention that some compilers do not support getch(). (Again Borland c needs the header file conio.h to be included)

Look at below example.

Example 5-6: example5-6.c

#include<stdio.h>
#include<stdlib.h>
main()
{
char choice;

while(1)
  {
  printf("\n\nMenu:\n");
  printf("1- Math Program\n2- Accounting Program\n");
  printf("3- Entertainment Program\n4- Exit");
  printf("\n\nYour choice -> ");
  choice=getch();
  switch(choice)
    {
     case '1' : printf("\nMath Program Runs. !");
              break;
     case '2' : printf("\nAccounting Program Runs. !");
              break;
     case '3' : printf("\nEntertainment Program Runs. !");
              break;
     case '4' : printf("\nProgram Ends. !");
              exit(0);
    }
  }
}

In above example we have rewritten example 5-1. This time we have used getch() function instead of scanf() function. If you test scanf based example you will see that it does not have any control on entered answer string. If user inserts an invalid choice or string (a long junk string for example), it can corrupt the screen. In getch function, user can insert one character at a time. Program immediately gets the character and tests it to see if it matches one of the choices.

In this example we have omitted optional "default" section in “switch...case”. Pay careful attention that in scanf based example we used to get an integer number as the user choice while here we get a character input. As a result previously we were using integer variable in “switch section” and integer numbers in “case sections”. Here we need to change the “switch … case” to match the character type data. So our switch variable is of char type and values being compared are used like '1', '2', '3', '4' because they are character type data ('1' character is different from the integer value 1).

If user presses an invalid key while loop will continue without entering any of "case" sections. Invalid key press will be omitted and only valid key presses will be accepted.

"continue" statement

Continue statement can be used in loops. Like break command "continue" changes flow of a program. It does not terminate the loop however. It just skips the rest of current iteration of the loop and returns to starting point of the loop.

Example 5-7: example5-7.c

#include<stdio.h>
main()
{
while((ch=getchar())!='\n')
  {
   if(ch=='.')
     continue;
   putchar(ch);
  }

system("pause");
}

In above example, program accepts all input but omits the '.' character from it. The text will be echoed as you enter it but the main output will be printed after you press the enter key (which is equal to inserting a "\n" character) is pressed. As we told earlier this is because getchar() function is a buffered input function.

Exercises

1. Write a program that reads input until enter key is pressed ('\n' is found in input) and prints number of alphabets and number of spaces in the input string. Use getchar() for reading input characters from console. ( example output: alphabet: 34 spaces: 10 )

2. Write a program that reads input and replaces below characters with the character that comes in front of them. Then writes the output on your screen.

a -> c
f -> g
n -> l
k -> r

Use getchar() and switch...case to write the program.